1. Field of the Invention
The present invention concerns a fan control for air conditioning units. More particularly, the herein described invention pertains to an economical control for a multi-speed fan for use in an air conditioning unit including an economizer.
2. Prior Art
Since the rise in the cost of energy, commercial structures, office buildings and retail stores, and other large air handling equipment users have investigated methods of saving energy relative to fan energy consumed in circulating air. Air handling requirements of an air conditioning system are designed to meet the highest requirements of the load or structure. Since the greatest power demand for cooling often occurs during the summer the air handling system must be designed for maximum summer conditions. Thus, during most of the year, the flow of conditioned air through the air handling units may be much greater than necessary. This increased capacity requires a greater initial equipment investment and potentially increased maintenance costs over the operating lift of the system. The operation of the air handling unit at higher capacity then necessary consumes considerable energy in powering fans and may add to the overall cooling load if the fan motor is located in the area cooled.
One method of decreasing air handling energy consumption at less than peak demand is to install load shedding equipment to cycle various air handling devices. Load shedding systems, however, involve motor and belt wear due to frequent cycling and uneven air distribution based on which air handlers are energized. In order to avoid the problems of load shedding, multiple speed fans have been used as an energy reduction device. By operating a system with multiple speed fans it is possible to match the volume of air being circulated with the air conditioning load.
It has been found advantageous to utilize air conditioning systems with economizers. The economizer as used herein is incorporated into an air conditioning unit to allow outdoor ambient air to be drawn into the unit for circulation to the enclosure. It is desirable to circulate outdoor air to the enclosure when its temperature and humidity are such that cooling of the enclosure may be accomplished without operating the refrigeration circuit of the air conditioning unit. The utilization of an economizer requires that sufficient air be circulated such that the air being drawn into the unit through the economizer is circulated to the enclosure and a sufficient volume of air is circulated back to the air conditioning unit. Under some conditions a power exhaust fan, return air fan or a discharge opening may be utilized such that the return air from the enclosure is discharged to the ambient. Hence, any cooling created in this mode of operation is the substitution of cool outdoor ambient air for the existing indoor air.
The present described invention concerns a rooftop type air conditioning unit having an economizer for drawing ambient air into the enclosure when appropriate. The herein invention also includes a multiple speed fan. To optimize energy consumption the fan is operated at high speed whenever ambient air is of sufficiently low temperature to provide cooling to the enclosure even though it is necessary to simultaneously operate refrigeration circuits to effect additional cooling. The fan is operated at low speed saving fan energy when the outdoor ambient temperature is high and the return air from the enclosure to the unit is being reconditioned by operation of the refrigeration circuits and recirculated to the enclosure.
When the temperature differential between the desired temperature and the temperature of the enclosure reaches a predetermined level an override is provided to operate the fan at high speed regardless of the economizer position or temperature of the outdoor air.